Fertilization:

IP3 and Egg Activation

A sequence of successive stages during Xenopus egg activation
in response to local injection of IP3. The IP3 was injected
at the right of the oocyte. Red indicates high levels of calcium;
blue indicates low levels. A caclium iondicator dyew is used to
assess calcium levels within the egg.

Egg activation
in most species is thought to involve a series of similar signal transduction
steps after sperm binds to a receptor protein on the egg/oocyte
surface. One of the steps that is thought to be crucial for subsequent
release of calcium from internal stores (typically, the endoplasmic reticulum, or ER)
is the production of inositol
triphosphate (IP3) from PIP2 residing in the egg plasma membrane.
The cleavage of PIP2 into IP3 and diacylglycerol
(DAG) results in numerous "downstream" events ã
IP3 is thought to stimulate local release of calcium ions from
the ER, resulting in the "calcium activation wave" as
a transient rise in calcium sweeps across the egg. The movie sequence
shown here, compiled from published still micrographs by Drs.
Carolyn Larabell and and Richard Nuccitelli (Larabell, C. and
Nuccitelli, R., 1992. Inositol lipid hydrolysis contributes to
the Ca wave in the activating egg of Xenopus laevis. Dev. Biol.
153, 347-355) demonstrates that local injection of exogenous IP3
into a Xenopus oocyte can result in in the generation of a calcium
wave.